Transfusion associated-and so called "community acquired"-hepatitis non-A, non-B is evidently caused by an agent that is, quite distinct hepatitis A and hepatitis B viruses. Recently, a sequence derived from this agent was cloned (Choo, Q.L., et al., Science, 244: 259-262, 1989) . Studies indicate that the agent causing hepatitis C is a positive stranded RNA virus-hepatitis C virus (HCV). At the present time, screening for HCV is based on the presence of antibodies in the sera or plasma of infected individuals to the recombinant HCV peptide, clOO-3, in sera or plasma. This peptide represents a non-structural viral component. Seroconversion (i.e. , the presence of clOO-3 antibody) does not occur for a long period of time (from about six weeks to possibly more than one year) after viral inoculation. Therefore, the feasibility of using PCR to detect HCV-specific sequences during this long window period was investigated. Using PCR in combination with reverse transcription and a pair of nested primers, we have developed a specific and sensitive (to a single copy level) diagnostic method for the detection of HCV sequences in clinical samples.